For obvious reasons, it’s important to know whether your watch is in a solid gold case or whether it is merely gold-filled or gold plated [“gold-filled” consists of a base metal such as brass sandwiched between two thin layers of gold]. The only way to be absolutely sure whether your watch case is solid gold, of course, is to take it to a competent and reputable jeweler and have it tested. But many watch cases are marked in such a way that you can usually figure it out if you know what to look for. Here are some pointers:
If the case is solid gold, it will often have a mark stating the gold content, such as “14K” or “18K”. Some [especially early American] case makers unscrupulously marked gold-filled cases as “14K” or “18K”, supposedly indicating that the cases were 14 or 18-karat gold-filled, so it is always best if the case also says something like “Warranted US Assay” after the karat marking. Again, when in doubt, have it professionally tested.
Some, especially European, watches express the gold content as a decimal. Pure gold is 24K, so a 18K watch would have “0.750” stamped on it and a 14K watch would have “0.585” stamped on it.
If a watch is only gold-filled it will often state that it is such. “Rolled gold” and “rolled gold plate” are similar terms that mean it is not solid gold. Note that a “14K Gold Filled” case is still just gold-filled.
A gold-filled case will often state how many years the gold is warranted to wear. Any time you see a period of years [“Guaranteed 20 years, “Warranted 10 years,” etc.] you can be sure the case is gold-filled and NOT solid gold. Keep in mind that an unusually heavy gold-filled case can sometimes produce a false reading when tested for gold content, and a solid gold case will NEVER be marked with a number of years it is warranted to wear. It is not uncommon to see a case marked “warranted 25 years” that is being sold as “solid gold” by a [hopefully] ignorant seller, and an informed buyer needs to be aware what he or she is actually buying.